how not to talk down to people

It's a lot easier to belittle someone than it is to imagine yourself in their shoes, but that is what you have to start doing if you want to stop being condescending. In most cases people who act like this do so because they have a deep-seated fear of rejection themselves so they reject before they are rejected. Most importantly you have to be willing to change because a half-hearted attempt is doomed to fail. Listen and accept what your wife has to say without rejecting her thoughts and opinions.